Is this unusually low compression?

281pony · Jul 12, 2004

a couple months ago my car was running like crap, and before we did a tune-up (new plugs, cap/rotor, ect) we did a compression test.

i was consistantly getting 120-125 in the cylinders we checked. is that really low? it seems like it should be around 170?

i have honestly never beat on the car. just seems like lately power has been on a steady decrease whenever i get on it a little. the engine has 101k almost on it.

just wondering how bad/low 125 is? guess it's time to get my new shortblock

ECU5.0 · Jul 12, 2004

i dont think that is low enough to have to buy a new engine. probably a different problem...like fuel filter

HISSIN50 · Jul 12, 2004

as long as they are all close, that is what matters. there are enough ways to do the compression test, that you can end up with different absolute numbers with each method. so we go with the consistancy amongst cylinders.

a leakdown test is more revealing. also if your compression goes, you start to see it in other areas - giving you a clue.

my two cents.

281pony · Jul 12, 2004

it has been burning more oil then normal. it burnt about a quart every 900-1k with the last oil change i did.

im just worried based upon the rest of this cars driveline that it was severely beaten and now im seeing the consequences of it. i just replaced the transmission due to bad syncro's, clutch was fried after 4k, throwout bearing wasn't replaced, transmission was loose, ect. alot of halfass stuff has been fixed at my expense.

with that track record, and the fact i got the car with the timing set at 31 degree's, fuel pressure 74 at idle, hole in the airfilter. i'm just thinking it's not running like it used to for a reason.

it has brand new plugs, fuel filter, cap/rotor, all the basic things.

im just thinking with 125 compression, possibly lower now, where is the line when you say its only going downhill? i'm saving my money for a new shortblock, heads, and a cam already, just wondering if i'm going to need this sooner rather then later.

HISSIN50 · Jul 12, 2004

with all of the other issues you have, i would hold off. as long as the motor performs alright...... most stangs burn some oil. a quart every 1500 miles is not unusual. i would hold off and once everything else is good, address the motor.

but we are all different, and only you can see and know how the car feels.

was the timing locked in at 31*? sounds like your previous owner might have had the shadetree dyno (cant remember which sub forum i saw that in). LOL.
if you are really concerned with the motor, a dry and wet compression test, done on a warm motor, all plugs out, throttle at WOT, would be the most accurate (the differences in technique i mentioned earlier). then do the same, but a wet compression test for comparison. and a leakdown test will tell much more than either of those.
my two cents, good luck with it all.

408stroker5.0 · Jul 12, 2004

That is low number's for a 5.0 with only 101k on it. You should be around 140 & up.... Hell I had 205,000KM on my old 5.0 and before I pulled it out for the 408 I did a compression test and she averaged 165-170. 125 isn't terribly bad, but a 5.0 with that amount of k's on her SHOULD have more. Chances are this car has had a rough life before you got it by the sounds of it and oil changes and care had been negleted. A hole in the air filter is NOT good and once again show's the lack of care for this engine. Chances are your ring's are shot which could explain the lower then average compression and lack of power. I would save your money and go for a nice rebuild, crate, or stroker build, your call on that. Check what Hissin5.0 said as well.... do a couple of different compression test's and see what you get.
BTW what's your oil pressure like??

281pony · Jul 12, 2004

thanks 408, thats what i was looking for. just a reference for what a well treated 5.0 should be at with 100k.

my oil pressure has been pretty consistant sitting at the notch above halfway on the stock gauge.

im planning to buy a new 306 shortblock, thumper heads, and a crower cam and finish off a h/c/i car with all the bolt-ons. later on if i want a blower or a stroker ill do it whenever the urge/money is there.

for now the car runs good enough, but it's been slipping so i wanted to see if the compression was a strong cause of it. im hoping it can get me 5k more miles without taking a **** on me. it's been going downhill pretty rapidly compared to how it was when i first got it. power seems less and less, burning more oil, ill have to do a real compression check and see what it is.

thanks for the suggestions and information

Is this unusually low compression?

281pony

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ECU5.0

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HISSIN50

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281pony

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HISSIN50

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408stroker5.0

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281pony

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Is this unusually low compression?

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